Originally, it's a barrier that prevents or slows fire from spreading. Some buildings have internal walls that separate them into sections that resist fire spreading across them. If you are inside such a buildingm you'll be able to recognize the firewall as it will have special doors that are either kept closed or have special latches that cause them to automatically close if the alarm goes off or the temperature gets too high.

Vehicles can also have firewalls. The divider between the passenger compartment and the engine compartment of a typical car is called a firewall because it's designed to keep fire and other dangers from the engine from getting to the passengers.

Computer firewalls are named by analogy to physical ones. They are systems which block certain traffic from crossing a network connection. A firewall will have a set of rules saying "this can cross over" and "this can not cross". The specifics of those rules can vary considerably depending on what they are meant to protect against.